Μηχανιστική μελέτη φωτοδιάσπασης και φωτομετάθεσης του αιθυλοβενζολίου με υπολογισμούς CASSCF/CASPT (Master thesis)

Αναστασόπουλος, Σωκράτης


The aim of the present thesis was to study the photochemistry/photophysics of ethylbenzene (PhCH2CH3) and in particular the photodissociation of the C-CH3 bond of the ethyl group, as well as that of the C-H bond of the methylene group. We have twospecific targets: (1) to give an interpretation of the experimental results of the photodissociation of ethylbenzenea, and (2) to compare the photochemistry with that of the benzylsilane analogue (PhCH2SiH3), latter showingthe formation of photometathesis products(photo-Fries)band to discuss possible reasons for their differentiation. The detection of conical intersections and their role in the formation of photo-Fries products and other possible products is a key issue of the study. In the context of this dissertation the quantum-chemical methods CASSCF/CASPT2 and basis set 6-31G(d) were used and their reliability was demonstrated. A complete mapping of the dynamic energy surfaces of the S0 (ground state), S1 (first singlet excited state), T1(first triplet excited state)and T2(secondtriplet excited state) was made along the C-C and C-H bond length coordinate. The critical points of the minima, the transition states (adiabatic transitions) of S0, S1 and T1, the S1/S0 conical intersections (non-adiabatic transitions), the intermediates and final products, as well as the VRI-points (ValleyRidgeInflection - points) have been determined. The uniform structure of the S1/S0 conical intersections of the benzyl systems(PhCH2CH3,PhCH2SiH3, PhCH2SiMe3) has been shown;this is an isosceles triangular pyramid which is defined by the two ortho-carbons of the benzene ring and the adjacent carbon of the methylene group, while on top of thepyramidlies the group which is going to be cleavaged/displaced and is almost perpendicularto the ipso-carbon atom. The calculations help to give areasonable interpretation of the experimental results: excitation of ethylbenzene with 248-nm laser light populates the S1 state (FC, Franck-Condon) which lies 4.8 kcal/mol lower than the conical intersection CoIn(S0/S1) and 3.6 kcal/mol higher than the S1min level;this justifies the low internal conversion(IC) efficiency (measured 25% slow component of radical production)a between the S1 and S0 states. The remaining 75% (measured fast component of radical production)aseems to be following an ISC (intersystem crossing) which leads to a high vibrational T1 state ('hot' T1) and finally to dissociation. Dissociation via T1min is ineffective due to high activation energyEα(T1(TS))and the corresponding negligible rates we calculated. The dissociations we assumed to occur through pre-fulvenic conical intersection geometries, either directly via superfast isc(S1/T1) or through doubles crossingsisc(S1/T2) and then CoIn(T2/T1). The study makes also some new product predictions originated from the same conical intersection CoIn(S0/S1) and the followingscheme summarizes all thesepathways: Pathway a(IC) leads back to the parent compound S0min, pathway c leads to the bicyclic product P1, pathway b to the ortho-photo-Fries product P2 and pathway d to the radical ground state. Also, a VRI point was found related to the phenomenon of a bifurcation which is occurring in the neighborhood of the bicyclic product P1 (ground state). It was confirmed that there is a linear dependence of the activation energy Eα to the reaction heat ΔHr and the latter on the bond dissociation energy D0, in both the ground and the excited states (T1,S1). It appeared that the cleavage of the methyl group is more likely compared to the cleavage of the hydrogen of the methylene group, thisbeing a consequence of the lower bond dissociation energy of the C-CH3bond compared to that of CH-H. Finally, an explanation for the experimental failure to detect photo-Fries products for ethylbenzene is given: we assume this is because the responsible conical intersectionCoIn(S0/S1) is energetic higher than the FC and S1min levels, unlike the homologous benzylsilane where the corresponding CoIn(S0/S1) lies lower.
Institution and School/Department of submitter: Πανεπιστήμιο Ιωαννίνων. Σχολή Θετικών Επιστημών. Τμήμα Χημείας
Subject classification: Φωτοχημεία
Keywords: Φωτοχημεία,Επιφάνειες δυναμικής ενέργειας,Αιθυλοβενζόλιο
URI: http://olympias.lib.uoi.gr/jspui/handle/123456789/29118
Appears in Collections:Διατριβές Μεταπτυχιακής Έρευνας (Masters)

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